DESIGN AND STRUCTURAL ANALYSIS OF ORGANIC SEMICONDUCTOR BASED ELECTRONIC DEVICES FOR BIO-MEDICAL APPLICATIONS

Abstract

Organic Electronics depicts a complete paradoxical working mechanism as compared to the conventional devices and provides an alternating platform to obtain flexible, lightweight, transparent, foldable device designs with satisfactory performance. These low-cost organic devices can be further realized in various applications which is quite impressive and motivating for the research community. The present work focuses on the designing of OTFT (Organic Thin Film Transistor), OLED (Organic Light Emitting Diode), and OPD (Organic Photo Diode) for performance improvement and analyses their usage for biomedical applications. The device structure plays a prominent role in comprehending its performance parameters. Therefore, these devices are analysed with respect to their structure to ameliorate the device performance. The conventional planar OTFT illustrates a decent current-voltage characteristics but these suffers from the limitations of grain boundary effect, large electric field requirements, and channel length constraints. A shorter length for the channel is very much needed to attain high performance TFTs. It helps in providing high drive current, fast switching, high transconductance, and good on-off current ratio (Ion/Ioff). However, the reduction in channel length is quite difficult due to lithographical constraints in planar TFTs. Therefore, a short channel vertical channel OTFT is proposed which contains channel length in nano meter (nm) range. The device exhibits a significant improvement in terms of threshold voltage (Vt), maximum drain current (IDmax) and saturation mobility (μsat) in comparison to planar device. The proposed vertical device (D5) consists of a vertical channel between source and drain that too with a ditch-like structure. The device witnessed a significant improvement of 44 and 24 times for IDmax and μsat in comparison to the planar device (D1). Furthermore, five other vertical- channel device structures (D2, D3, D4, D6 & D7) are compared. Out of these structures, the proposed structure (D5) shows remarkable performance in terms of IDmax (528 μA) and μsat (80.8 cm2/V.s). It exhibits an increment in IDmax by 44, 2.38, 6.4, 4.4, 44 and 3.59 times as compared to the other devices; D1, D2, D3, D4, D6 and D7, respectively. Additionally, the proposed device exhibits a significant improvement in μsat which is higher by 24.5, 2.67, 8.32, 5.21, 73.45 and 3.6 times in comparison to D1, D2, D3, D4, D6 and D7, respectively. Further, to better understand the facts associated with high performance of proposed device, the vertical and horizontal cutline analysis is performed. A horizontal cutline is drawn at 5 nm below the source electrode and parameters; band energies, potential distribution, total current density, vii electron/hole concertation are examined. The result of this analysis shows the strong gate control and small channel length as the primary attributes for performance enhancement. Further, focusing on the structural aspects, an organic LED structure is proposed to design an effective light emitter for bio-medical applications. In recent years, The OLEDs have been integrated spectacularly in large panels for display and biomedical applications. Different layers used in the OLED architecture witnessed a prominent role in its performance. Herein, a CGL (Charge Generation Layer) is utilized to enhance the carrier concentration inside the EML (Emissive Layer) in OLED. This layer consists of HAT-CN (hexaazatriphenylene- hexacarbonitrile) material for electron and TAPC (1,1-bis[(di-4- tolyamino)phenyl)]cyclohexane) for holes generation. In the proposed structure (L6), the CGL layer is incorporated outside of the EML which significantly enhances the device performance. The proposed design is compared with other five CGL/non-CGL based OLED devices (L1, L2, L3, L4 and L5). The proposed device showcased the luminescence and current values of 3636.3 cd/m2 and 0.44 A, respectively. The obtained luminescence is about 16.8, 2.3, 1.7, 3, 1.6 times improved than that of L1, L2, L3, L4 and L5, respectively. Thereafter, an in-depth internal analysis is performed to better analyse the behaviour of device. The proposed device is analysed and compared with other mentioned devices in terms of Langevin recombination rate, electron concentration, hole concentration, band energy, total current density, electron affinity, hole QFL (Quasi-Fermi Level), conduction current density, potential distribution, and electron/hole mobility. The results depict that the CGLs generate excess charge carriers and enhance the carrier concentration within the EML layer. Consequently, the device luminescence is enhanced and validates the Langevin’s theory. Additionally, the energy levels of CGLs show a close match with the other adjacent layers, which further enhances the carrier movement from the electrodes. Thereby, the current density within the device is also improved. The OLED device witnessed a potential use as the light source in biomedical applications. However, the detection must be carried out by the OPD. Additionally, it is desired to realize both the devices on a single substrate for smaller size and efficient sensing. Hence, an efficient CGL based organic photo diode is designed and analysed to work as a suitable light sensing element. Organic photodiodes have emerged as better alternative owing to their flexibility, reduced dark current, thin active layer, and tuneable spectral responsivity during the last decade. In the proposed device, the CGL is incorporated to enhance the exciton interaction that leads in generating more free charge carriers and thereby augment the conduction. In this work, viii the proposed device (P2) is compared with other four non-CGL (P1) and CGL (P3, P4 and P5) based devices. Another important aspect with CGL based devices is the position of this layer with respect to other layers. In the proposed device (P2), the CGL is placed outside the active layer in such a manner that HAT-CN is placed near acceptor and TAPC near donor layer. Consequently, the proposed device depicts a remarkable improvement and showcased fair enough values for photocurrent (134.2 nA) and dark current (10.2 nA). The photocurrent of P2 is increased by 34 times in comparison to the reference device (P1). Also, on comparing with other CGL based devices; P3, P4 and P5, the photocurrent of P2 is enhanced by 1.6, 1.4 and 9 times, respectively. Further, the thickness optimization and internal analysis of the proposed device are also performed to better comprehend the role of layers in the OPD. The parameters; electron/hole concentration, electron current density, potential, conduction/valence band energy and electron QFL are observed and quantitatively compared. The results highlight an enhanced carrier generation due to exciton interaction with active layers and CGLs, leading to a high current generation. The integration of OLED and OPD devices is the best suitable combination for point-of-care diagnostic in the field of sensor applications. Also, the OTFT can be used as the driver. This work proposes the methodology for bio-medical applications using organic LED and photo- diode to make an efficient, flexible, and low-cost sensor. The fluorescence-based detection methodology is presented and utilized in three medical applications; corona virus detection, heart rate monitoring, and ovarian cancer detection. Primarily, the materials for both OLED and OPD devices are modified to make these devices suitable for the detection of SARS-COV- 2 (Severe Acute Respiratory Syndrome Coronavirus-2) virus RNA (Ribonucleic Acid) inside human saliva sample. In OLED, the materials; DPVBi (4,4'-Bis(2,2-diphenylvinyl)-1,1’- biphenyl) & BCzVBi (4,4'-Bis(9-ethyl-3-carbazovinylen)1,1'-biphenyl) are chosen for the EML layer so that the device can emit a light of 470 nm wavelength. In case of OPD, the materials; C60 and CuPc are considered for acceptor and donor, respectively. When the emission wavelength falls on the saliva sample, fluorescence emission takes place. For a normal person, this emission is in the range of 490 nm and correspondingly OPD produces a current of 37.2 mA magnitude. However, due to the presence of SARS-CoV-2 RNA, the emission from the infected person is in the range of 525 nm, resulting in OPD current value of 63.5 mA. Thus, a healthy person can be differentiated from a Covid-19 infected patient.Thereafter, in this work, a green color-based OLED is utilized for heart rate measurement. This OLED is based on the proposed device structure incorporating CGLs. The primary reason of choosing green color OLED is its wavelength which can be absorbed by the blood easily. Also, it has low penetration in the skin and thus provides better contrast signal. For OPD, the materials; C60 and CuPc are chosen for accepter and donor, respectively. The methodology is based on the fluorescence emission where green light with wavelength of 497 nm falls on the blood cells. Furthermore, a different intensity of the reflected wavelength is sensed by the photodiode. The methodology depicts that during the diastole, comparatively a higher current is produced by the OPD as the absoption by the blood cell remains less and reflection is more. On the contrary, the OPD provides low current values in the case of systole due to higher absorption of the green light as high volume of blood is available at this time. The proposed OLED exhibits current and luminescence values of 0.92 A and 8363.6 cd/m2, respectively and OPD demonstrates dark current and photocurrent of 10.2 and 174.5 nA, respectively. This methodology is also applied for multiple person’s and compared with some existing work in terms of excitation and detection wavelengths. The proposed OLED and OPD are also used for the detection of Ovarian cancer. As per the requirements, the QAD (4-(5,6-dimethoxy-1-benzothiophen-2-yl)-4-oxobutanoic acid) material is opted for EML (OLED) which is responsible for producing desired wavelength of 350 nm. This OLED generates fair enough current density and luminescence values of 37.02 mA/cm2 and 3636.38 cd/m2, respectively. Both these devices have the same structure as the proposed devices, only the material for specific layers are changed to match the need of the application. The OLED emits the target light beam of 350 nm which falls on the urine sample. Thereafter, the altered light beam is sensed by the photodiode and produces subsequent output current. For the healthy person, the reflection occurs at 420 nm and corresponding current attains the magnitude of 31.2 nA. Whereas, for a person having ovarian cancer, the emission spectra is in the range of 440 nm and subsequent current registers a value of 47 nA. Thus, the healthy human can be easily differentiated from a cancer patient. Furthermore, it is also shown that the proposed detection technique is applicable for multiple persons. The research conducted herein shows the effective utilization of organic devices for biomedical application. The investigation of OLED and OPD highlights their effectiveness to be used for non-invasive sensing applications. This further widens up the horizons for investigating organic devices in the field of biomedical applications.